The invention relates to a ceramic casting compound (slip), which includes a ceramic powder, an aqueous dispersion medium and a binder. A casting compound of this type is known from WO 94/07808. In addition to the casting compound, a process for producing the casting compound and a use of the casting compound are described.
A ceramic casting compound is used, for example, to produce a ceramic green sheet. A casting compound with an aqueous dispersion medium is preferred with a view to ecological compatibility. The ceramic casting compound which is described in WO 94/07808 and is suitable for producing a ceramic green sheet has as its binder an emulsion copolymer of an acrylate and/or methacrylate.
Ceramic green sheets are used to produce a ceramic multilayer body, for example a multilayer capacitor. To do this, a plurality of green sheets, which may have been metallized, are stacked on top of one another, are laminated, the binder is removed and the sheets are sintered together. Green sheets which are as thin as possible are used in order to reduce the overall size of the multilayer body. In addition, thin green sheets allow a capacitance of the capacitor to be increased in the case of a multilayer capacitor.
For a green sheet to be reliably processed, a low modulus of elasticity (high flexibility) and, at the same time, an appropriate tensile strength are advantageous mechanical properties of the green sheet.
It is an object of the invention to provide a ceramic casting compound having an aqueous dispersion medium, with the aid of which it is possible to produce a thin ceramic green sheet which, moreover, has sufficient elasticity and appropriate tensile strength for further processing.
To achieve. the object, the invention describes a ceramic casting compound which includes a ceramic powder, an aqueous dispersion medium and a binder, characterized in that the binder includes polyurethane.
A suitable binder is polyurethane, which is used, for example, for the production of a polyurethane foam, an adhesive or a paint.
A chain of a polyurethane is highly flexible on account of its molecular structure. Moreover, crosslinking between the chains allows the green sheet produced with the aid of the polyurethane to have a tensile strength which is suitable for further processing of the green sheet. In this way, it is possible to produce a very thin green sheet which has the desired mechanical properties and therefore can be reliably processed further. A thin green sheet may in particular be processed on its own, i.e. without a support such as a plastic sheet. It is not necessary to specifically influence the mechanical properties of a green sheet, for example by incorporating a soft substituent of a polymer binder, in order to reduce its glass transition temperature. It is also possible to dispense with a plasticizer as a casting compound additive.
In a particular embodiment of the invention, the binder includes a copolymer. By way of example, the binder is an emulsion copolymer of a polyacrylate and a polyurethane. Cellulose, polyacrylamide, polyvinyl alcohol and/or styrene may also form a further constituent of the copolymer.
A specific property of the binder (for example the solubility of the binder in water) can be altered in a controlled way by using a monomer with a suitable functional group.
A further advantage of the invention is that a binder made from polyurethane can be mixed with a further binder. This further binder may in particular include polyacrylate. In this case, it is particularly advantageous if the binder has a similar pH stability.
In particular, the casting compound has a binder with a molar weight from a range between 10,000 and 120,000 g/mol.
The binder is preferably in the form of a microsphere with a mean particle size from a range between 20 and 300 nm, a particle size of below 100 nm being possible in particular with a polyurethane. This particle size means that the polyurethane is readily soluble in water, thus ensuring high stability of the casting compound. Solid constituents remain uniformly distributed in the casting compound, i.e. there is no sedimentation (gravitational separation). Moreover, a small particle size of the binder makes it possible to achieve a high green density in a ceramic green body.
In a particular configuration, the casting compound has a pH from a range between 5 and 10. In this range, a polyurethane is stable, i.e. it does not tend, for example, to form agglomerates. There is no need for an additional stabilizer for the polyurethane.
The ceramic powder of the casting compound has at least one material which is selected from the group consisting of boron, carbon, oxygen, sulfur, nitrogen, lithium, sodium, potassium, beryllium, magnesium, calcium, strontium, barium, aluminum, gallium, indium, titanium, zirconium, bismuth and/or manganese. In particular, the ceramic powder includes barium titanate. Barium titanate is used primarily as a dielectric in multilayer capacitors.
To achieve the object, a process for producing a ceramic casting compound which includes a ceramic powder, an aqueous dispersion medium and a binder with polyurethane is also described. For this purpose, an aqueous dispersion of the ceramic powder and an aqueous dispersion of the binder are mixed.
A ceramic casting compound described here is used to produce a ceramic green body, for example a green sheet. The casting compound can be used in particular to produce a green sheet which has a thickness from a range between 1 and 30 xcexcm.
A thin green sheet which includes, for example, barium titanate as ceramic powder can be used to produce a multilayer capacitor of high capacitance and low overall height.
To summarize, the following important advantages are associated with a polyurethane as a binder for a ceramic casting compound:
A casting compound with an aqueous dispersion medium can be used in a ceramic production process.
When removing the binder from a ceramic green body which has been produced from the casting compound, complete, residue-free combustion (decarburization) of the polyurethane takes place.
In the case of a polyurethane, a glass transition temperature Tg is so low that the minimum film-formation temperature of the casting compound is not exceeded. This is beneficial with regard to a good mechanical property of a green sheet for further processing.
A polyurethane has a relatively high solubility in water. This leads to a stable casting compound with a high green density.
A polyurethane can readily be combined with another binder (multicomponent binder).
Monomers of a polyurethane can readily be subjected to copolymerization. It is thus possible to specifically influence a property of a binder at the molecular level.
With the aid of a ceramic casting compound which has a polyurethane binder it is possible to produce a very thin ceramic green sheet. The green sheet has mechanical properties which allow further processing of the green sheet (combination of flexibility and tensile strength).